Emergence of Blackcurrant Gall Mite (Cecidophyopsis Ribis) from Galls in Spring

The emergence of the blackcurrant gall mite, Cecidophyopsis ribis (Westwood), from galls on the blackcurrant cultivars Ben Lomond and Ben Tirran was monitored closely in 1995-1999 using miniature sticky traps. Emergence was preceded by swelling of the galls. First, 5% and 50% emergences varied from Julian day 74-112, 84-121 and 101-129 respectively in the different years but were virtually identical on the two cultivars, even though 'Ben Tirran' flowered on average 14 d later than 'Ben Lomond.' First emergence was often associated with the first day after 1 March with a maximum air temperature > 16°C. More satisfactory predictions of the seasonal timing of emergence were made by accumulated temperature sums above a threshold of 4deg;C from Julian day 46 (15 February). The average accumulated temperatures for first, 5% and 50% emergences were 122, 199 and 316 degree-days which gave mean errors in the predictions of 3.1, 1.3 and 7.2 d respectively. In 1998, the onset of emergence was delayed by a prolonged period of wet weather during which the internal tissue of the galls became necrotic, particularly on 'Ben Tirran.' The accumulated temperature model predicted the first, 5% and 50% emergences to within 4, 4 and 5 d respectively in 2000. There was great day-to-day variability in the mean number of mites that emerged. There was a positive correlation between the number of mites emerging and mean or maximum daily temperature but numbers were suppressed on days with rainfall. There was great variability in the timing of emergence of mites from individual galls and the variance of the number of mites emerging was related to the mean according to Taylor's power law. The emergence of mites had a strong diurnal rhythm, controlled by both light and temperature, with virtually no mites emerging between 2300 hours and 0930 hours. Galls contained several thousand motile mites and roughly double the number of eggs during the early part of the migration when the internal tissue of the galls was green and succulent. The internal tissue of the galls became progressively dry and chlorotic as the migration progressed and the numbers of mites and the emergence dwindled and eventually ceased. Implications for the control of the gall mite with acaricides are discussed.